Jet Streams run in a narrow belt at high altitude (above 12,000 m). They are predominantly westerly winds.

They speeds range from about 110 km/h in summer to about 184 km/h in winter.

These winds flow parallel to the Himalayas and due to their great height they divide the stream into two, one flowing to north and other flowing south of Himalayas.

The Jet Stream in Winter

Jet stream and western disturbace

The western disturbances are low pressure regions which originate in Mediterranean region and are carried with the Jet streams.

The jet stream which flows west from Mediterranean and Caspian region, along N latitude towards south of Himalayas is responsible for bringing the Western Disturbances.

This low pressure system as it moves towards India (with the westerlies) forces moisture laden winds from Arabian Sea to flow towards the land mass of Indian continent. For example, in the map below, near Rajasthan, winds are blowing anti-clockwise. In the northern hemisphere, such an anti-clockwise flow of air indicates a low pressure region. This low pressure is associated with the Western Disturbances. To normalize the reduced air pressure over Rajasthan, moist winds from the Arabian Sea seem to be blowing towards Rajasthan. Now when these moisture hits Himalayas it rises and causes precipitation. As a result, cloudy weather and rains will be witnessed in the green-shaded areas of central and northern India.

Western Disturbances are the principle source of rain during non-monsoonal months especially over Northwest India including Punjab, Haryana, Uttar Pradesh and Delhi. Their effect sometime extends up to Gangetic plains and Northeast India. They are also responsible for bringing snowfall in the higher reaches of Jammu & Kashmir, Himachal Pradesh and Uttarakhand.

In summer the western jet streams shift to the north of the Himalayas creating a low pressure region over Indian Peninsula which forces the monsoon (moisture laden) winds from the south to cause rainfall.

The tropical jet stream blows over the Indian Peninsula; approximately over 14° north during the summer months.

(vi) Define monsoons. What do you understand by “break” in monsoon?

Answer:

The prevailing wind direction in India is due to north-easterly winds which blow from sub-tropical high pressure belt of northern hemisphere. The monsoon, causes the seasonal reversal in wind direction so that the wind now flows from the seas and oceans in the south towards the land mass in north. Such a reversal due to monsoons causes most of the rainfall received in India.

Monsoon tends to have ‘breaks’ in rainfall that means that the entire season is inter-spaced with are rainy and dry spells.

The monsoon rains only come for a few days at a time and other days are rainless intervals.

(vii) Why the monsoon is considered a unifying bond?

The monsoon brings unifying cultural and economic effects over India even though the weather patterns vary widely across India.

During May, when the entire country us sweltering in summer heat, each and every citizen is waiting eagerly for monsoon rains.

The joy of monsoon is shared by entire country, especially the farmers who depend on the monsoon.

The monsoon determines the entire agricultural calendar. The many festivals celebrated across the country are tuned to the monsoons. Even though they are known by different names, they join the entire country in the spirit of celebration.

In the Indian landscape, its flora and fauna are highly influenced by the monsoon.